Posts Tagged ‘microwave oven’

What do wall warts, aka AC wall adapters, and microwave ovens have in common? Well, in previous blogs discussing microwaves, we saw how a microwave oven’s high voltage circuitry uses a transformer, diode, and capacitor to effectively convert AC voltage into DC voltage. Wall warts do much the same thing and in very much the same way.

If you will recall from our discussion of microwave ovens in the past few weeks, the transformer in a high voltage circuit transforms 120 volts into a much higher voltage, say 4000 volts, in order to make things work. The diode and capacitor within both the microwave and the wall wart are key to facilitating this magical act, but in the wall wart it happens at a much lower voltage, about 12 volts.

Last week we began exploring the inner workings of the wall wart. We discovered how its transformer converts the 120 volts emanating from your average wall outlet to the 12 volts required by most electronic devices. These voltages are shown at Points A and B in Figure 1 below. The fact that the voltage being put out results in waves of energy which alternate between a positive maximum value, zero, and a negative maximum value, makes it an unacceptable power source for most electronic devices. They require voltage that doesn’t alternate, and this is where the wall wart’s diode bridge and capacitor come into play.

Figure 1 – The Workings of the Wall Wart Transformer

The wall wart’s diode bridge consists of four electronic components, namely the diodes, which are connected together. This diode bridge goes a bit further than the single diode present in a microwave oven, because it doesn’t merely eliminate negative aspects of alternating voltage. It actually transforms negative voltage into positive voltage. The result is a series of 12 volt peaks as shown at Point C of Figure 1. In fact, we end up with twice as many voltage peaks, and this is important, as you’ll see below.

We still have the problem of zero voltage gaps to address. You see, over time the voltage at Point C of Figure 1 keeps changing between 0 volts and positive 12 volts. This can lead to problems, because many electronic devices require a consistent voltage of greater than zero to operate properly. For example, a light emitting diode (LED) might develop an annoying flicker, or you might end up hearing an irritating hum while listening to the radio. These annoyances are virtually eliminated by feeding voltage from the diode bridge into the capacitor, which gets rid of the zero voltage gaps between the voltage peaks.

Like a microwave’s capacitor, the one within a wall wart charges up with electrical energy as the voltage from the diode bridge nears the top of a peak. Then, as voltage begins its dive back to a zero value, the capacitor discharges its electrical energy to fill in the gaps between peaks. The result is the rippled voltage pattern at Point D of Figure 1. With the gaps filled in, the voltage is at, or close enough to, the 12 volts required to keep an electronic device operating properly when it is connected to the wall wart’s low voltage power cord.

Well, that’s it for our look at the wall warts that power our myriad of electronic devices. Next time we’ll switch to a totally topic and look at some of the basics of food manufacturing equipment design.

Ever been jolted with electric current? Like the time you’d just gotten out of the shower and went to plug in a lamp with damp hands? So what do you think the voltage was that caused that nasty biting feeling that resulted from your momentary lapse in good judgment?

Once, while operating a subway car at a railroad museum at which I was a member, I was inadvertently “electrocuted.” I went to turn on the lights inside the car, and unbeknownst to me the light switch was faulty. When I touched it I instantly became connected to the car’s 600 volt lighting circuit. With just a split second of contact the current passed through the tip of my right index finger, along my right arm, down the right side of my body, and out the tip of my big toe, finally exiting into the metal railcar’s body. The current actually burned a hole where it had exited through my boot. The experience was both frightening and painful, but fortunately did not result in any real injury. I was lucky that the current had bypassed my heart, because if it hadn’t, I might not be alive today.

That was 600 volts. Now imagine being jolted by the 4000 volts present in a microwave oven’s internal high voltage circuitry.

Last week we discovered how the high voltage circuit in a microwave oven converts the ordinary, everyday 120 volts alternating current (AC) present in our homes into a much higher voltage approximating direct current (DC). This is done by an internal component known as the capacitor. The capacitor is capable of storing large amounts of electrical energy, and this can result in microwave ovens presenting a danger even when unplugged.

A microwave oven capacitor is shown in Figure 1. If you happened to touch its wire terminals while it’s still charged, its power can rapidly discharge high voltage electrical current throughout your body. The electrical current from the capacitor can even stop your heart from beating, and this is exactly what caused the demise of a person featured on a soon to be released Discovery Channel program, Curious and Unusual Deaths. While being interviewed as an expert for the program, I was asked to explain this rather unique phenomenon of latent stored energy, and how it may present a threat.

Figure 1 – A Microwave Oven Capacitor

Remember, a microwave oven capacitor can remain charged with dangerous electrical energy for hours, even days, after the microwave oven plug is pulled from the wall outlet. The bottom line here is that you should not attempt to fix your microwave oven, unless you are trained and certified to do so.

Next week we’ll switch to a different topic, namely an electrical device known as a “wall wart.” That’s the black plastic adapter you plug into electrical outlets to power your cell phones, laptops, and other small electronics.

The world of electricity is full of mysteries and often unanticipated outcomes, and if you’ve been reading along with my blog series you have been able to appreciate and come to some understanding of a fair number of them. This week’s installment will be no exception.

Last week we looked briefly at the high voltage circuit within a microwave oven. We discovered that the circuit contains a transformer that raises 120 volts alternating current (AC) to a much higher voltage, around 4000 volts AC. The circuit then transforms the AC into direct current (DC) with the help of electronic components known as a diode and capacitor. Let’s take a closer look at how the diode and capacitor work together to make AC into DC.

Let’s follow an AC wave with the aid of a device called an oscilloscope. An oscilloscope takes in an electronic signal, measures it, graphs it, and shows it on a display screen so you can see how the signal changes over time. An AC wave is shown in Figure 1 as it would appear on an oscilloscope.

Figure 1 – Alternating Current Wave

You can see that each wave cycle starts with a zero value, climbs to a positive maximum value, then back to zero, and finally back down to a maximum negative value. The current keeps alternating between positive and negative polarity, hence the name “alternating current.”

Within the microwave oven’s high voltage circuitry the transformer does the job of changing, or transforming if you will, 120 volts AC into 4000 volts AC. This high voltage is needed to make electrons leave the cathode in the magnetron and move them towards the anode to generate microwaves.

But we’re not done with the transformation process yet. The magnetron requires DC to operate, not AC. DC current remains constant over time, maintaining a consistent positive value as shown in Figure 2. It is this type of consistency that the magnetron needs to operate.

Figure 2 – Direct Current

The microwave’s diode and capacitor work together to convert the 4000 volts AC into something which resembles 4000 volts DC. First the diode acts like a one-way valve, passing the flow of positive electric current and blocking the flow of negative current. It effectively chops off the negative part of the AC wave, leaving only positive peaks, as shown in Figure 3.

Figure 3 – The Diode Chops Off The Negative Part of the AC Wave

Between the peaks are gaps where there is zero current, and this is when the capacitor comes into play. Capacitors are similar to batteries because they can be charged with electrical energy and then discharge that energy when needed. Unlike a battery, the capacitor charges and discharges very quickly, within a fraction of a second.

Within the circuitry of a microwave oven the capacitor charges up at the top of each peak in Figure 3, then, when the current drops to zero inside the gaps the capacitor comes into play, discharging its electrical energy into the high voltage circuit. The result is an elimination of the zero current gaps. The capacitor acts as a reserve energy supply to fill in the gaps between the peaks and keep current continually flowing to the magnetron. We have now witnessed a mock DC current situation being created, and the result is shown in Figure 4.

Figure 4 – The Capacitor Discharges to Fill In The Gaps Between Peaks

The output of this approximated DC current looks like a sawtooth pattern instead of the straight line of a true DC current shown in Figure 2. This ripple pattern is evidence of the “hoax” that has been played with the AC current. The net result is that the modified AC current, thanks to the introduction of the diode and energy storing capacitor, has made an effective enough approximation of DC current to allow our magnetron to get to work jostling electrons loose from the cathode and putting our microwave oven into action.

You now have a basic understanding of how to turn AC into an effective approximation of DC current. Next week we’ll find out how this high voltage circuit can prove to be lethal, even when the microwave oven is unplugged.

My mom was a female do-it-yourselfer. Toaster on the blink? Garbage disposal grind to a halt? She’d take them apart and start investigating why. Putting safety first, she always pulled the plug on electrical appliances before working on them. Little did she know that this safety precaution would not be enough in the case of a microwave oven. Let’s see how even an unplugged microwave can prove to be a lethal weapon and, yes, we’re going to have to get technical.

Last week we talked about the magnetron and how it needs thousands of volts to operate. To get this high of a voltage out of a 120 volt wall outlet–the voltage that most kitchen outlets provide–the microwave oven is equipped with electrical circuitry containing three important components: a transformer, a diode, and a capacitor, and just like the third rail of an electric railway system these items are to be avoided. If you decide to take your microwave oven apart and you come into contact with high voltage that is still present, you run the risk of injury or even death. But how can high voltage be present when it’s unplugged? Read on.

First we need to understand how the 120 volts emitting from your wall outlet becomes the 4000 volts required to power a microwave’s magnetron. This change takes place thanks to a near magical act performed by AC, or alternating current. In the case of our microwave components, specifically its diode and capacitor, AC is made to effectively mimic the power of DC, or direct current, the type of current a magnetron needs. This transformation is made possible through the storage of electrical energy within the microwave’s capacitor.

Next week we’ll examine in detail how this transformation from AC to DC current takes place, as seen through a device called an oscilloscope.

The evolution of cooking methods has been interesting indeed, from the open fires of primitive man, who must have decided at some point that cooked meat tasted better than raw, on to wood fired stoves, fossil fuel-based cooking, whether coal, propane, or gas, and let’s not forget electric range tops. Standing on its own in the modern kitchen is the microwave oven. What will be next? The space age food pill dispensing stations of the futuristic cartoon family, The Jetsons?

We’ve been talking about resonant cavity magnetrons and the purpose they serve within a microwave radar system. We also learned about Dr. Percy Spencer’s discovery and how microwave radar transmissions emanating from a magnetron can cook food, not to mention melt candy bars.

Figure 1- Microwaves Melt Candy Bars and Cook Food

Although the technologies used in microwave radars and microwave ovens are similar, they do have important differences. It would be both unsafe and impractical to install microwave radar systems into kitchens. Radiation emitted from radar wave guide lacking proper containment would bounce aimlessly around the kitchen, posing a threat to human safety. You see, microwaves don’t know the difference between our bodies and the food we wish to cook. They’ll heat up human tissue just as readily as a bowl of chicken soup. Another issue is that runaway microwaves lose much of their effectiveness through their aimless bouncing about, and much of it would not be directed to the food itself. Dr. Spencer would learn how to corral that energy, making microwave cooking a commercial success.

The biggest problem for Dr. Spencer to overcome was containment of the microwaves. They needed to be directed towards food in order to efficiently heat them. His first microwave oven was a metal box containing an opening at the top into which a magnetron wave guide could be inserted. This would then introduce microwaves into the box, and the metal construction of the box would safely contain them. The safety issue had now been resolved because the waves couldn’t escape, they would simply bounce around inside the box and most of their energy would be transferred into any food placed inside.

Dr. Spencer’s employer, the Raytheon Corporation, produced the first commercial microwave oven in 1954, and it was appropriately named the “Radarange.” It was huge, almost six feet tall, and weighed in at about 750 pounds! Hardly something that could fit into a home kitchen. Despite its massive size, the Radarange wasn’t all that powerful and couldn’t compete against the compact countertop microwave ovens in use today.

It wasn’t until 1967 that technology improved enough to give us the smaller, more efficient units affordable to consumers. This improvement involved using a newly developed semiconductor device called a “diode” within the high voltage electric circuitry that powers the magnetron. We’ll learn more about these technologies in our next post.

Also in our next post, we’ll see how high voltage circuits can pose electrocution hazards in a way you‘d never expect. I discussed one of these instances in my recent appearance on The Discovery Channel program, Curious and Unusual Deaths, soon to be aired.

Ever discover a melted candy bar in your pocket? You immediately start to think about the sources for the heat that had caused the mess. Did you stand too close to the stove, were you outside in summer heat too long, or did you simply sit on it? Or was it perhaps caused by being in proximity to a whirring device, something which does not seem to generate any heat at all? If you’ve been reading along with us, you know what device I’m talking about.

Last time we talked about an effect known as cavity resonance and how a sound is created, much like a musical note, when we blow across the top of a glass pop bottle containing some air space. Our breath causes the air molecules to bounce in and out of the bottle’s cavity, producing the sound. Microwave technology works in much the same way, making use of an electronic device called a resonant-cavity magnetron. But instead of generating a musical tone, like our breath does over the bottle, the magnetron produces short wavelength radio waves, known as microwaves, and it was initially developed to generate these microwaves for radar systems. So, how does the magnetron work?

The magnetron contains a series of cavities arranged in a circle, their openings pointed towards the center as shown in Figure 1. Engineers determined that when a high voltage, say 4000 volts, is applied to the magnetron, it begins to boil off electrons through a filament, called a cathode, located at its center. Once free of the cathode, the electrons want to flow to a part of the magnetron called the anode. This is because the cathode is positively charged and the anode is negatively charged, and as we know, electrons like to flow from positive to negative. The anode is also the part of the magnetron containing the cavities, and we’ll see the significance of this in a moment.

Figure 1 – Interior View of a Resonant-Cavity Magnetron

Before the electrons can take their desired straight path to the anode, they are deflected by powerful magnets located on either end of the magnetron. These magnets force the electrons to move in a circular pattern over the openings in the cavities. Like the air molecules passing over the top of a pop bottle when you blow across it, the electrons move over each cavity opening in the magnetron, creating not musical tones, but microwaves. The microwaves are then collected from the magnetron using an antenna and directed along a tube called a wave guide. The microwaves leave the wave guide when they are transmitted by radar systems. The radar system then transmits the microwaves towards moving objects they wish to track. These tracked objects are as diverse as airplanes, ships, and weather patterns. See Figure 2.

Figure 2 – Microwave Radar Transmission

So, how does the microwave oven fit into our discussion? In 1946 an engineer by the name of Percy Spencer was working on a radar magnetron for the Raytheon Corporation, a producer of electronic technology for industry and defense. During the course of his work he unexpectedly exposed himself to microwaves from a wave guide, and he couldn’t help but notice that the candy bar in his pocket had melted. Putting two plus two together, he realized the microwaves had caused the candy bar to heat up. Dr. Spencer further experimented and came to the conclusion that microwaves can cook foods far more quickly than conventional ovens, and the modern day kitchen appliance was soon born.

Next time we’ll look at how Dr. Spencer’s microwave cooking discovery was developed into the microwave oven we find in most kitchens. We’ll also see how even an unplugged microwave oven can pose an electrocution hazard, as I explained in the Discovery Channel program I was recently featured on, Curious and Unusual Deaths.

In the world of inventions it happens with some regularity that an invention to do one thing unexpectedly leads to a device that does something completely different. Take for example Edison’s phonograph. At the time, he was working on an invention to record the dots and dashes of Morse code telegraph messages so they could be sent out repeatedly without an operator having to tap them out each time and possibly making mistakes while doing so. Little did he know that this would lead to the evolvement of the phonograph and recording industries.

Another invention “by mistake” took place when the resonant cavity magnetron, originally developed for use with microwave radar, led to the development of the microwave oven. Last week we talked about how long wave radar, the first type of radar to be developed, was effectively used by the British to repel enemy air attacks during World War II. But long wave radar was large and cumbersome to employ, and it soon evolved into an improved version, the shorter wave, or, as we know it, microwave radar. So what is this resonant cavity magnetron that led to its creation? A pop bottle can give us a clue.

Blow across the top of an empty glass pop bottle (or soda bottle, depending on the part of the country you’re from) and a familiar resonant sound results. The sound is created by an effect known as cavity resonance, and other bona fide musical instruments make use of this phenomenon to produce musical sounds. How this works is that where a cavity exists, when air molecules are introduced into it, the molecules are caused to resonate in and out of the cavity many times per second. This creates a sound at a certain frequency, that frequency depending on the shape and dimensions of the cavity, as well as the size of its opening.

The resonant cavity magnetron, or magnetron for short, is actually a high powered vacuum tube that operates in a very similar fashion to a pop bottle, or any other musical device making use of a cavity, but instead of using air molecules to generate sound waves, it uses electrons to generate short wavelength radio waves, called microwaves. The magnetron contains a series of cavities that are arranged in a circle with the openings pointing inward towards the center, as shown in Figure 1.

Figure 1 – Interior View of a Resonant-Cavity Magnetron

Next week we’ll see what this interesting looking device has in common with the simple act of blowing air across the top of a pop bottle and what this all has to do with microwaves.

Pop some corn, defrost meat, quickly heat a frozen dinner. What modern day miracle is capable of doing all this and more? The microwave oven, of course. They’ve become an integral part of modern kitchens and our lifestyle. Did you ever wonder how they came to be? We’ll have to turn back the hands of time over 70 years to the dark days of World War II to find out.

WWII began when Adolph Hitler’s Nazi regime conspired with Joseph Stalin’s Soviet Russia to invade Poland in September, 1939. From there, German forces went on to sweep across much of continental Europe, and even as far as North Africa and the Middle East. In the face of these and future war maneuvers, Great Britain remained our ally and a significant force to be dealt with.

Although Hitler was intent on bringing the British Empire to it knees by means of aerial bombardment, he did not succeed. He failed to take into account the fact that the British Royal Air Force Fighter Command had a unique advantage over his German Luftwaffe– or Air Force to those of you not up on your German. The advantage was called RADAR, an acronym for RAdio Detection And Ranging. Thanks to this proprietary technology, the British Fighter Command was able to detect incoming German warplanes well in advance of their arrival over British soil, and this provided British pilots with enough time to scramble their fighter planes and get a jump on their attackers. Although horribly outnumbered, the Royal Air Force planes managed to inflict heavy casualties on the Luftwaffe. In fact, this Battle of Britain, as it came to be known, marked the first defeat of Hitler’s military forces during the war.

So how does this modern marvel of RADAR work? Simply stated, it transmits radio waves from an antenna. When these waves strike an object, say an approaching German airplane, some waves are reflected back to a special receiver antenna. The reflected radio waves are then converted by the radar system into a “blip” on a viewing screen. A radar operator stands at the ready, trained in the art of interpreting radar blips. He or she is quickly able to assess the approaching object’s position, direction of travel, and speed of approach.

Although the ideas that led to the development of radar had been around for some time before WWII, it wasn’t until the late 1930s that engineers began developing it for air defense purposes. They first developed what was called a long wave radar system, referring to the fact that the radio waves transmitted had a long wavelength, about 1.5 meters. The British, at the top of their game, had anticipated the Nazi threat and installed an array of these on their east coast by 1940. Although long wave radar worked well enough during the Battle of Britain, it required massive, land-based antennas to operate, which proved to be cumbersome. It also couldn’t give operators a good idea of how many planes were approaching.

Then, just months before the beginning of the Battle of Britain, British researchers developed an ultra top secret device they dubbed a “resonant-cavity magnetron,” or magnetron for short. This magnetron was capable of producing short radio waves, in the range of 1 to 10 millimeters, and these came to be known as microwaves.

The technology mastered in the magnetron opened the door to an improved radar system, one which became mobile. These systems were more easily installed on aircraft, ships, and land vehicles. Microwave radar was also found to be better at detecting small objects than its massive predecessor technology. Unfortunately, microwave radar couldn’t be put to use in time for the Battle of Britain, but it did play a pivotal role in the defeat of Axis, that is the opposing side’s, forces during the remainder of WWII.

Next time we’ll take a closer look at the resonant cavity magnetron and find out how microwave radar led to the development of the famous oven that bears its name.

Last week I said we’d talk about ground fault circuit interrupters (GFCI), but before we get to that let me provide a little background on how I recently came to be a subject matter expert for The Discovery Channel.

The program I was asked to contribute to is titled, “Curious and Unusual Deaths,” where I will be discussing a variety of tragic occurrences, including deaths by electrocution. It’s actually slotted to be a series of half-hour segments presenting, you got it, some of the strangest, most grizzly, tragic, or simply avoidable deaths from across the globe and throughout time.

The episodes use actors to recreate the incidents, which take place in a variety of locations, from the home to the workplace. After presenting a graphic portrayal of the unfortunate event, experts, including myself, provide an explanation of the science behind it all. Cinematic recreations and high quality computer generated imagery are used to recreate how the deaths occurred as well as illustrate the experts’ explanations.

It all started when I was contacted by a researcher for the program. She had discovered me through this very blog site, the Engineering Expert Witness Blog, and was impressed with my diverse background, which as my loyal readers know covers both mechanical and electrical engineering. She also liked the fact that I’ve worked professionally within many different industries. Needless to say, I was flattered.

She went on to explain that she was interested in discussing with me the technical aspects of four particularly unusual but actual incidents that had proven to be deadly to those involved in them. The deaths were due to a variety of factors, from a steam pipe failure, to contacting high voltage electronics through a seemingly benign unplugged appliance, to succumbing to carbon monoxide poisoning produced by a propane heater, and finally, the one I thought to be most unusual of all, a death caused by touching a hotel room door that was, unbeknownst to the person inserting the key in the lock, charged with stray electrical current from an unlikely source. Sound interesting? Look for the series to air in the near future to find out more.

We discussed the incidents by phone for awhile, and then a week later I received another call inviting me to fly up to Toronto, Canada, to be videotaped as an expert for the series. Now, although I do not have a ham bone in my body, I do enjoy sharing my knowledge of technical things with others, so I gladly took her up on the offer.

Tune in next week to read the insider’s track on the making of a quality television series.